Cystic Fibrosis (CF) is the most common fatal genetic disease in humans (Boat, T. et al. Cystic fibrosis. In: The Metabolic Basis of Inherited Disease, C. Scriver, A. Beaudet, W. Sly, and D. Valle, eds. (McGraw Hill, New York, 1989), 2649-2860). Based on both genetic and molecular analysis, a gene associated with CF was isolated as part of 21 individual cDNA clones and its protein product predicted (Kerem, B-S. et al. Science 245:1073-1080 (1989); Riordan, J. et al. Science 245:1066-1073 (1989); Rommens, J. H. et al. Science 245:1059-1065 (1989)).
U.S. Ser. No. 07/488,307 describes the construction of the gene into a continuous strand, expression of the gene as a functional protein and confirmation that mutations of the gene are responsible for CF. (See also Gregory, R. J. et al. Nature 347:382-386 (1990); Rich, D. P. et al. Nature 347:358-363 (1990)). The co-pending patent application also discloses experiments which showed that proteins expressed from wild type but not a mutant version of the cDNA complemented the defect in the cAMP regulated chloride channel shown previously to be characteristic of CF.
The protein product of the CF associated gene is called the cystic fibrosis transmembrane conductance regulator (CFTR) (Riordan, J. et al. Science 245:1066-1073 (1989)). CFTR is a protein of approximately 1480 amino acids made up of two repeated elements, each having six transmembrane segments and a nucleotide binding domain. The two repeats are separated by a large, polar, so-called R-domain containing multiple potential phosphorylation sites. Based on its predicted domain structure, CFTR is a member of a class of related proteins which includes the multi-drug resistant (MDR) P-glycoprotein, bovine adenyl cyclase, the yeast STE6 protein as well as several bacterial amino acid transport proteins (Riordan, J. et al. Science 245:1066-1073 (1989); Hyde, S. C. et al. Nature 346:362-365 (1990). Proteins in this group, characteristically, are involved in pumping molecules into or out of cells.
CFTR has been postulated to regulate the outward flow of anions from epithelial cells in response to phosphorylation by cyclic AMP-dependent protein kinase or protein kinase C (Riordan, J. et al. Science 245:1066-1073 (1989); Frizzell, R. A. et al. Science 233:558-560 (1986); Welsh, M. J. and Liedtke, C. M. Nature 322:467 (1986); Li, M. et al. Nature 331:358-360 (1988); Hwang, T-C. et al. Science 244;1351-1353 (1989); Li, M. et al. Science 244:1353-1356 (1989)). Difficulties can be encountered when attempting to purify CFTR or other membrane-associated proteins because a purification process may not result in a functional protein.